|
The search for cause(s) of cancer has been going on for centuries.
Early researchers said that cancer was a natural result of
aging. As cells degenerated, it was believed that some simply
became malignant. Others said cancer was hereditary, and investigations
into genetics began. Then some began to consider chemical
links while still others questioned whether viruses or bacteria
were at fault. Finally, the "irritation" theory became popular,
and researchers began trying to identify irritants - such
as tobacco and coal tar - that would cause cancer in laboratory
animals. Ultimately, though, cancer experts were forced to
confront the fact that although all these factors
|
|
might be involved, none of them invariably
cause cancer. Not every animal or person
exposed to an irritant or a particular chemical in the
laboratory developed cancer, nor did all elderly people
or everyone with a family history of cancer get it.
As a result, scientists had to abandon the theory that
cancer had a single cause.
|
However, despite the fact that there is yet no absolute agreement
among the cancer research community in terms of what actually
causes cancer, scientists are certain that
many factors can be linked to cancer. These factors, including
many other possible causes of cancer suggested by cancer researchers,
are believed to be "cancer risk factors." These risk factors
include eating habits, lifestyle, living or working environments,
genetics, and many others. Following are some major cancer
risk factors identified by researchers with the support of
scientific statistics:
Smoking
|
|
Cigarette smoking alone is directly
related to at least one-third of all cancer deaths annually
in the United States. Cigarette smoking is the most
significant cause of lung cancer and the leading cause
of lung cancer death in both men and women. Smoking
is also responsible for most cancers of the
|
larynx, oral cavity, and esophagus. In addition, it is highly
associated with the development of, and deaths from bladder,
kidney, pancreatic, and cervical cancers. Tobacco smoke contains
thousands of chemical agents, including 60 substances that are
known to cause cancer (carcinogens).
The health risks with cigarette smoking are not limited to
smokers. Exposure to environmental tobacco smoke significantly
increases a nonsmoker's risk of developing lung cancer. Environmental
tobacco smoke is the smoke that nonsmokers are exposed to
when they share air space with someone who is smoking.
Diet
|
|
The life style factor that has received
the most attention in recent years is diet. Evidence
suggests that about one-third of the cancer deaths each
year that occur in the United States are related to
dietary factors. These include types of food, preparation
methods, portion size, variety, and overall caloric
balance.
|
A high-fat diet has been associated with an increased risk
for cancer of the prostate, endometrium, and colon and rectum.
It is believed that a high-fat diet is a cancer promoter,
with numerous theories to explain the effects of excess fat.
For instance, excess fat seems to be involved in the production
of free radicals,
which play a role in many types of cancer. A high-fat diet
also increases the flow of bile acids into the intestine,
which can promote colon cancer.
Study results suggest that certain food additives, as well
as preparation methods, can either cause or promote cancer.
Even some so-called natural methods of preserving foods are
not considered safe. For example, pickled, cured, and smoked
products appear to promote stomach cancer, possibly due to
nitrites used in curing as well as to other compounds produced
during smoking and pickling. The decrease in gastric cancer
incidence is largely due to modern refrigeration and a reduction
in pickled, cured, and smoked food products.
Genetics
|
|
By definition, cancer is really a disease
of genes. Genes are very small molecules in our cells,
which determine almost everything in our body. Genes
that control the genetics and heredity of each cell
are strung like beads on a necklace along the cell's
DNA in the cell nucleus. In a benign or malignant tumor,
several of the genes regulating these processes are
abnormal (mutated). Abnormal genes may be inherited
or damaged by carcinogens, viruses, errors in cell division,
and as yet unknown factors.
|
A number of the most common cancers, including breast, colon,
ovarian, and uterine cancer, recur generation after generation
in some families. In addition, certain genetic factors may
predispose those affected to specific cancers. A few rare
cancers, such as the eye cancer, retinoblastoma, and a type
of colon cancer, have been linked to specific genes that can
be tracked within a family.
Although it is helpful to know the role that our genetic
heritage may play as a possible cause of cancer, scientists
believe that environmental influences and our behaviors may
outweigh the risks inherent in our family tree.
Occupation and Environment
Scientists have long been aware of the linkage between one's
health conditions and their occupation and environment.
People who have direct contact to carcinogenic agents in
the workplace are at the highest risk for developing cancer.
For example, a recent study suggests that people with brain
cancer are more likely to have worked in certain occupations
than similarly aged people without brain cancer. Many cancer-causing
chemicals have been identified and many of them are banned
from manufacture in the United States.
|
|
More recently, investigators have identified
a link between the environment and skin cancer. The
environmental factor is something we depend on for our
life: sunlight. Scientists have found that ultraviolet
light causes mutations of genes, producing a carcinogenic
effect. Now, we not only know that
|
tumors may appear years after the damaging effects of sunlight,
but also the risks from exposure to ultraviolet light are greater
for light-skinned people. Statistics show that in the U.S. alone,
about a million new cases of skin cancer (basal
and squamous cell carcinomas) occur annually, rivaling the
incidence of all other types of cancer combined.
The common body surfaces that are exposed to carcinogens
are the skin, nasal passages, and lung. The primary internal
body surface that has contact with carcinogens is the urinary
bladder.
Infectious Agents
|
|
Because viruses can invade and alter
cells' genetic material, viral infections are implicated
in some cancers. The Epstein-Barr virus, for example,
is associated with Burkitt lymphoma, a tumor found mainly
among children in Africa. The hepatitis B virus is responsible
for much of the liver cancer around
|
the world. The highest rates of hepatitis B infection in the
world is in China, Taiwan, Japan, and Thailand with equally
high rates of liver cancer in these countries. The human papilloma
virus that causes genital warts has been shown to play an important
causative role in cervical cancer. The human T-cell leukemia
virus, a close relative of the virus that causes acquired immunodeficiency
syndrome (AIDS), is associated with a cancer known as Kaposi
sarcoma and some types of Non-Hodgkin lymphomas.
Cancer risk factors are not limited to those listed above.
There are still other risk factors such as ethanol use, use
of certain medications, hormones, and reproductive and sexual
behavior. With further scientific research, more cancer risk
factors will be identified in the future.
In summary, cancer is caused by both external (chemical,
radiation, and viruses) and internal (hormones, immune conditions,
and inherited mutations) factors. Causal factors may act together,
or in sequence, to initiate or promote carcinogenesis.
|
